Long term isothermal reliability of copper wire bonded to thin 6.5 µm aluminum

In long term reliability evaluations of Spansion memory products built using copper (Cu) wire bonding in lieu of gold (Au) wire for package-to-die interconnection, results indicated acceptable reliability performance of the copper-aluminum (Cu-Al) bond. Some differences, however, were observed when compared to gold-aluminum (Au-Al) bonds used as a control. In order to determine if these differences represented a true reliability concern, a series of experiments were run on a variety of process technologies (from 200nm to 65nm) to determine wear-out failure mechanisms of these Cu bonds and their associated apparent activation energies. Isothermal reliability tests at three temperatures (150 °C, 175 °C, and 200 °C) were performed using uncoated 0.9 mil Cu wire (with 0.9 mil Au wire as a control) bonded to functional flash die from a variety of process technologies. All bonding was done to 6.5 µm thick Aluminum-Copper (Al-0.5% Cu) bond pads. Bond shear and wire pull values were measured at each readpoint and the experiments were continued through extended long term readpoints to insure that products were stressed until failure. The primary failure mechanism identified was interfacial cracking between the copper bond and the intermetallic layer, starting at the rim of the bond. The apparent activation energy computed for the Cu-Al bond interfacial cracking was 0.70 eV. Subsequent calculations of expected product lifetime in various usage models using this Ea show that the Cu wire bonding provides more than adequate reliability lifetime for all expected product usage scenarios.